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<br />Precipitation received from November through March <br />is generally in the form of snowfall. Thunderstorms are <br />prevalent in July and August and are often localized, <br />with high-intensity rainfall. Prolonged droughts and <br />lesser periods of deficient moisture may be interspersed <br />with periods of abundant precipitation. <br />There are periods of extremely cold winter and hot <br />summer temperatures in the basin. Extremes range from <br />winter lows of -600F. in Montana to summer highs of <br />1200F. in Nebraska, Kansas, and Missouri. The basin <br />regularly experiences over-IOO-degree temperatures in <br />summer and below-zero temperatures in winter over <br />most of its area. <br />One of the climatic factors influencing much of the <br />culture and industry in the basin, particularly agri- <br /> <br />FIGURE 6 <br />MEAN LENGTH OF FREEZE-FREE PERIOD (DAYS) <br /> <br />LEGEND <br /> <br /> <br />c=J UNDER 30 <br />c::::J 30- 60 <br />c::::J 60- 90 <br />G::J 90-120 <br />~120-150 <br />c::J 150-180 <br />I.-J OVER 180 <br /> <br />Winds in the basin are the rule rather than the <br />exception, particularly in the plains area. Average wind <br />velocities of 10 miles per hour are prevalent over much <br />of the basin. In the plains area, strong winds accom- <br />panied by snow sometimes creat "blizzard" conditions <br />dangerous to both man and livestock. High winds <br />occasionally prevail during periods of high temperatures <br />and deficient moisture that can destroy crops and <br />desiccate rangeland within a few days. Parts of the basin, <br />particularly the south and east, are subject to cyclonic <br />and tornadic winds that occasionally do considerable <br />damage. <br />Most climatic forces are not amenable to change, but <br />modern technology has done much to enable man to <br />better cope with the extremes that affect his environ- <br />ment and culture. Special farm and range conservation <br />practices, irrigation, and air conditioning in homes and <br />factories are examples. Generalizations of the basin's <br />climate, however, are not always applicable to specific <br />areas or to their seasonal advantages. Many people are <br />attracted to parts of the basin because of the favorable <br /> <br />12 <br /> <br />culture, is the continuous period of above-freezing <br />conditions. The freeze-free period is defined as the <br />average number of days each year between the last <br />320F. temperature in the. spring and the first 320F. <br />temperature in autumn. While the freeze-free period <br />does not completely define the growing season for all <br />crops and grasses, it is a general indicator of the most <br />favorable period. It is also an indicator of the period <br />during which special precautions must be taken against <br />freeze-up. Figure 6 shows the average freeze-free period <br />for the basin which, for the non-mountainous areas, <br />ranges from about 90 to 180 days. Mean total hours of <br />sunshine annually are favorable for agriculture and a <br />healthful environment, ranging from about 2400 to <br />3200 hours, as illustrated by figure 7. <br /> <br />FIGURE 7 <br />MEAN TOTAL HOURS OF SUNSHINE (ANNUAL) <br /> <br />LEGEND <br /> <br /> <br />c=J UNDER 211-00 <br />C]21100-2600 <br />C:J 2600-2800 <br />_2800-3000 <br />[::::::J 3000-3200 <br />IIiiiii OYER 3200 <br /> <br />climate, and particularly the cool, dry, crisp days of <br />summer are a tourist attraction in the western and other <br />mountainous areas. <br /> <br /> <br />Livestock Losses From Blizzards Are Not Uncommon <br />